1. Field of the Invention
The present invention relates to a fuel vapor supply control apparatus in a lean-burn internal combustion engine, which supplies an intake system with a fuel vapor produced in, e.g., a fuel tank etc in accordance with an operating state of the lean-burn internal combustion engine, and particularly to an apparatus for detecting a concentration of the fuel vapor.
2. Related Background Art
In an engine which has hitherto generally been used, the fuel is injected out of a fuel injection valve to an intake port, and a combustion chamber is previously supplied with a uniform air-fuel mixture of the furl and the air. In the thus constructed engine, an intake passageway is opened and closed by a throttle valve interlocking with an operation of an accelerator.
A quantity of the intake air supplied to the combustion chamber of the engine (which is resultantly a quantity of a uniformly mixed gas of the fuel and the air) is controlled by opening and closing the throttle valve, thereby controlling an output of the engine.
According to the technology based on the so-called uniform combustion described above, a large intake negative pressure is produced with a throttle operation of the throttle valve, and a pumping loss becomes large, resulting in a decline of efficiency. By contrast, a technology known as a so-called stratified charge combustion, wherein the throttle valve is throttled small, the fuel is supplied directly to the combustion chamber, a combustible air-fuel mixture is thereby made to exist in the vicinity of a spark plug, and an igniting property is enhanced by increasing an air/fuel ratio of the portion concerned. According to this technology, when in a low-load state of the engine, the fuel injected is supplied in dispersion around the spark plug, and the stratified charge combustion is executed with the throttle valve substantially fully opened. The pumping loss is thus reduced, and a fuel consumption is enhanced.
Th internal combustion engine capable of performing the above-described stratified charge combustion sequentially assumes, for example, when changed from the low-load to a high-load, combustion states such as the stratified charge combustion, a weak stratified charge combustion, a uniform lean-burn and a uniform combustion.
The stratified charge combustion is, as explained before, that the air/fuel mixture exhibiting a low air/fuel ratio is made to exist in the vicinity of the spark plug, and is stratified between this mixture and a gas at another portion.
The weak stratified charge combustion has a smaller degree of its being stratified than the stratified charge combustion.
The uniform lean-burn has a uniformity of the fuel and the air but is small in terms of a ratio of the fuel.
The uniform combustion has a uniform mixture of the fuel and the air and a high ratio of the fuel.
Further, there might be a case where a swirl is formed in the air/fuel mixture of the injected fuel when the above-described stratified charge combustion is conducted, and when the lean-burn is effected. That is, an intake port is provided with a swirl control valve (SCV), and an aperture of this valve SCV is controlled, thereby controlling an intensity of the swirl. As a result, the combustibility is enhanced with a small amount of the fuel supplied.
Incidentally, there is known an apparatus for controlling a supply of a fuel vapor in a lean-burn internal combustion engine (Japanese Patent Application Laid-Open Publication No.4-194354) constructed such that the fuel vapor (vapor) from the fuel tank etc is temporarily accumulated in a canister, and the fuel vapor accumulated is supplied to an intake system in accordance with an operating state of the internal combustion engine.
According to this technology, a purge control valve is provided within a fuel vapor oriented purge passageway through which the canister for adsorbing the fuel vapor is connected to an intake passageway. Then, the purge control valve is controlled so as to obtain a proper fuel purge quantity (a quantity of the fuel vapor introduced into the intake passageway, which is hereinafter simply referred to as a purge quantity) (e.g., so as to supply the fuel vapor if a load of the engine is large) in accordance with the operating state of the engine.
In a lean-burn region, however, the air/fuel ratio can not be detected at a high accuracy, and hence a real situation is that there is no index for controlling the fuel purge quantity.
To be more specifically, according to the prior art, the air/fuel ratio sensor such as an oxygen sensor is disposed in the exhaust passageway, and an actual air/fuel ratio is detected based on an output signal of this sensor. Then, the fuel injection quantity etc is feedback-controlled so that the air/fuel ratio of the air/fuel mixture becomes a target air/fuel ratio specially calculated. At this time, even when the air/fuel ratio becomes rich with an execution of purging the fuel vapor, the feedback control is performed so that the air-fuel mixture comes to have the target air/fuel ratio. The oxygen sensor described above, however, makes such a detection that the target air/fuel ratio (A/F) is in the vicinity of, e.g., a stoichimetric air/fuel ratio (A/F=14.5). In the case of the lean-burn where the air/fuel ratio is larger than this stoichimetric air/fuel ratio, it is impossible to highly accurately detect a change in the air/fuel ratio due to the purging. Besides, it is impossible to precisely calculate an index (e.g., a concentration of the fuel vapor) for controlling the purge quantity of the fuel vapor which has hitherto been calculated from the output of the air/fuel ratio sensor in the prior art.
Therefore, in the above-described lean-burn region, an accuracy of calculating a purge quantity declines if the air/fuel ratio is not detected, and if the detected air/ratio does not exhibit a high accuracy in the case of controlling a fuel vapor supply quantity. Then, if the fuel vapor supply controlling apparatus is controlled based on the purge quantity determined from the negative pressure, there might be a possibility in which an accidental fire and a surge occur when the vapor is rich.